Automobile spark ignition and diesel engines have valve train systems, including valves, cams and rocker arms, which present special lubrication concerns. It is extremely important that the engine oil protects these parts from wear. Further, it is important for engine oils to suppress the production of deposits in the engines. Such deposits are produced from non-combustibles and incomplete combustibles of hydrocarbon fuels (e.g., gasoline, diesel fuel oil) and by the deterioration of the engine oil employed.
Engine oils use a mineral oil or a synthetic oil as a base oil. However, simple base oils alone do not provide the necessary properties to provide the necessary wear protection, deposit control, etc., required to protect internal combustion engines. Thus, base oils are formulated with various additives, for imparting auxiliary functions, such as ashless dispersants, metallic detergents (i.e., metal-containing detergents), antiwear agents, antioxidants (i.e., oxidation inhibitors), viscosity index improvers and the like to give a compounded oil (i.e., a lubricating oil composition).
Meanwhile, combustion gas produced in a gasoline engine or a diesel engine is mostly exhausted from the exhaust pipe as exhaust gas, but a portion of the combustion gas leaks through a gap between piston and cylinder to blow into the crankcase as blow-by gas. Since the blow-by gas contains nitrogen oxide gas (NO.sub.x) of high concentration, it oxidizes and deteriorates an engine oil (crank case oil) charged in the crank case. Recently, since lightweight motor cam are required in view of saving energy (improving fuel-efficiency), the crank cases of the engines are apt to be made small. Therefore, the amount of the engine oil charged in such a small crank case is lowered and the relative amount of nitrogen oxide introduced into the crank case as blow-by gas per the amount of the charged engine oil is increased. Consequently, the deterioration of engine oil is apt to be promoted. In addition to the above-described reason, since recent engines are often used to satisfy severe demands (high speed and/or high power), the concentration of nitrogen oxide in blow-by gas is further increasing.
As is described above, factors in the deterioration of engine oils are more and more increasing. Once an engine oil is deteriorated, its acid value is increased and sludge is deposited in the crank case. Therefore, engine oils are desired to be protected from the deterioration as far as possible. Hitherto, in order to inhibit the oxidation and the deposition of sludge, additives such as oxidation inhibitors (e.g., zinc dithiophosphate, oxidation inhibitors of phenol type) and detergent-dispersants (e.g., ashless dispersants, metal-containing detergents) have been added to engine oils. However, since recent engines are apt to be used under severe conditions as is described above, the known additives cannot satisfactorily prevent engine oils from deteriorating. Therefore, it is desired to take measures to meet this problem.
The use of phenol sulfides in lubricating oils is well known in the art.
U.S. Pat. No. 3,236,770 discloses a lubricating oil having a dialkylthiocarbonate, and either a metal dialkyl dithiophosphate; a basic alkaline earth metal sulfonate; or a basic alkaline earth metal alkyl phenol sulfide.
U.S. Pat. No. 3,240,705 discloses a 2,2'-trithiobis-(4-halo-6-alkylphenol).
U.S. Pat. No. 4,302,683 discloses an organo-molybdenum complex prepared by reacting a hydrocarbyl substituted thio-bis-phenol with a molybdenum source.
U.S. Pat. No. 4,764,294 discloses an engine oil having a base oil, a metal phosphate, a metal carbamate, and a para alkyl hydroxy-aryl sulfide.
The use of dithiocarbamates in lubricating oils is well known in the art.
U.S. Pat. No. 3,876,550 discloses lubricating compositions containing an alkylene bis(dithiocarbamate), as an antioxidant, and a substituted succinic acid as a rust inhibitor. The alkylene dithiocarbamate is represented in the patent by the formula R.sup.1 R.sup.2 N--C(S)--S-alkylene-S--C(S)--NR.sup.3 R.sup.4. Example 5 of the patent describes a crankcase lubricant containing a viscosity improver, an ashless dispersant and methylene bis(dibutyldithiocarbamate). The patent further teaches that the composition may also contain various other additives, for example, detergents, dispersants, viscosity improvers, extreme pressure agents, antiwear additives, etc., as well as other oxidation inhibitors and corrosion inhibitors and cites an extensive list of extreme pressure agents, corrosion inhibitors and antioxidants, including zinc salts of phosphorodithoic acid.
The use of methylene bis(dibutyldithiocarbamate) as an oxidation inhibitor in lubricating oils, in combination with other ingredients, is also disclosed in U.S. Pat. Nos. 4, 125,479 and 4,880,551.
U.S. Pat. No. 4,879,054 is directed to cold temperature greases and teaches using dithiocarbamates such as Vanlube 7723, i.e., 4,4'-methylene bis(dithiocarbamate), in such greases to provide extreme pressure antiwear properties. Examples 13-18 describe using Vanlube 7723 and triarylphosphate as replacements for lead naphthenate and zinc dithiophosphate.
The use of dithiocarbamates as extreme pressure antiwear additives is also taught by U.S. Pat. Nos. 4,859,352, and 4,648,985 teaches that the combination of dithiocarbamates with zinc dithiophosphate and copper salts of carboxylic acid provide lubricants with extreme pressure properties.
U.S. Pat. No. 4,383,931 discloses using lubricating oils containing an oil-soluble molybdenyl bis-.beta.-diketonate in combination with zinc dithiophosphate. Methylene bis(dihydrocarbyldithiocarbamates) are used as ashless antioxidants and extreme pressure agents.
U.S. Pat. No. 4,501,678 discloses using lubricating oils containing an alkyl thiocarbamoyl compound and either a molybdenum thiocarbamoyl compound or a molybdenum organophosphorodithioate.
U.S. Pat. No. 4,609,480 discloses using lubricating oils containing an alkyl thiocarbamoyl compound and a 1,3,4-thiadiazole compound. The oils can also contain, among other things, sulfurized oxymolydenum organophosphorodithioates.